This invention relates to improved amplifier circuits and, in particular, to an improved single ended push-pull amplifier formed of field effect transistors having triode operating characteristics and including a differential amplifier as the pre-amplifying stage thereof.
The use of push-pull amplifiers is well known for providing high power amplification with minimum distortion. The single ended push-pull (SEPP) amplifier is one form of a push-pull amplifier whereby the power efficiency has been greatly increased. A typical SEPP amplifier is described in U.S. Pat. No. 2,936,345 and is comprised of triodes connected in push-pull configuration and being supplied with input signals by a differential amplifier which also is formed of triodes. This SEPP amplifier suffers from inherent disadvantages: the use of the vacuum tubes is undesirable and satisfactory operation has not been readily achieved by substituting conventional transistorized elements for the triodes; and cross-over distortion is quite noticeable.
If a solid-state version of this prior art SEPP amplifier is constructed so as to be manufactured in integrated circuit configuration, the corss-over distortion remains as a highly undesirable feature.
It has been found that if field effect transistors (FET's) are used as the push-pull amplifying components, the problem of cross-over distortion can be solved by using a variable current source in cooperation with the differential amplifying stage of the SEPP amplifier.